Inverters comprise a DC line that is connected to a DC power supply, and a pair of switching elements connected in series between the high potential side and the low potential side of the DC line, for example. The connection point of the pair of switching elements is connected to an AC load electrically.
There is a case where a large voltage (surge voltage) is instantaneously applied in synchronism with switching the switching elements of an inverter. The amount of the surge voltage depends on various conditions such as a switching speed of the switching elements.
Conventionally, it is suggested, for an inverter adopting power semiconductor switching elements (IGBT, MOSFET, etc.) that control a current by the amount of gate charge, to change the switching speed by switching the gate charge current only in a small current area, or to extend a life time of switching elements by changing the gate resistance of switching elements in accordance with the load amount.
As the speed of switching of the switching element becomes faster, the amount of change in current flowing through a switching element becomes larger, and the surge voltage also becomes larger. In light of this phenomenon, a method for controlling the gate charge current in accordance with the voltage of the DC power supply has been suggested.
In addition, it is known that if a contactor provided to the DC line shuts off the DC line to protect the DC power supply, or the DC power supply is opened due to a failure during regenerative operation of an inverter, the voltage of a smoothing capacitor provided at the DC power supply in/out unit of the inverter abruptly increases for a short time. In this situation, if a response time of a control means detecting an over-voltage is insufficient, it is suggested to provide a means for blocking a PWM signal if the voltage exceeds a predefined voltage.
However, for an inverter capable of adjusting the speed of switching, a threshold for detecting an over-voltage has to be set as equal to or less than the value obtained by subtracting the maximum surge voltage for the maximum speed of switching from the maximum rated voltage of a capacitor and a semiconductor element. Accordingly, the difference between the maximum voltage to be applied to a switching element and the threshold becomes small (over-voltage detection margin) during normal operation, and it may result in a malfunction when detecting an over-voltage. To ensure an over-voltage detection margin, it is necessary to lower the maximum speed of switching to switch the surge voltage. This may increase switching losses in an inverter.